Representative embodiments are directed to vacuum pump systems for evacuating enclosed chambers such as processing chambers. In particular, representative embodiments are directed to vacuum pump systems that include a scroll pump.
A scroll pump is a type of pump that includes a stationary plate scroll having a spiral stationary scroll blade, an orbiting plate scroll having a spiral orbiting scroll blade, and an eccentric driving mechanism to which the orbiting plate scroll is coupled. The stationary and orbiting scroll blades are nested with a radial clearance and predetermined relative angular positioning such that a series of pockets are simultaneously defined by and between the blades. The orbiting plate scroll and hence, the orbiting scroll blade, is driven by the eccentric driving mechanism to orbit relative to the stationary plate scroll about a longitudinal axis of the pump passing through the axial center of the stationary scroll blade. As a result, the volumes of the pockets delimited by the scroll blades of the pump are varied as the orbiting scroll blade moves relative to the stationary scroll blade. The orbiting motion of the orbiting scroll blade also causes the pockets to move within the pump head assembly such that the pockets are selectively placed in open communication with an inlet and outlet of the scroll pump.
In a vacuum scroll pump, the motion of the orbiting scroll blade relative to the stationary scroll blade causes a pocket sealed off from the outlet of the pump and in open communication with the inlet of the pump to expand. Accordingly, fluid is drawn into the pocket through the inlet. The inlet of the pump is connected to a system that is to be evacuated, e.g., a system including a processing chamber in which a vacuum is to be created and/or from which gas is to be discharged. Then the pocket is moved to a position at which it is sealed off from the inlet of the pump and is in open communication with the outlet of the pump, and at the same time the pocket is contracted. Thus, the fluid in the pocket is compressed and thereby discharged through the outlet of the pump.
Moreover, vacuum scroll pumps are often used to remove air from chambers where the air may contain water vapor as a result of humidity. In this case, the water vapor in the air being exhausted may condense as the gas is compressed. If the amount of water vapor in the gas is relatively large, the saturation temperature of the gas being a function of both pressure and temperature, the saturation temperature will eventually exceed the actual gas temperature, at which point water will form as condensate of the gas between the blades of the scroll pump. This water can corrode components of the pump, and can absorb gases being pumped which can cause problems in the operation of the pump, etc.
To prevent condensation of gas inside a vacuum scroll pump, ballast gas (air or dry nitrogen, for example) is directed into the compression stage through a gas passageway at a location near but not at the downstream end of the compression stage. The ballast gas dilutes the gas being worked by the vacuum scroll pump in the compression stage. The added gas load also increases the temperature of the gas. The combination of these two factors reduces saturation pressure of the gas stream below the actual gas temperature and condensation of water vapor is prevented. In addition, the use of ballast gas applies to the vapors of other substances which will take liquid form at the combinations of pressure and temperature that can exist within a vacuum pump, e.g., various organic solvents.